When an aircraft comprises a planned flight trajectory (which has been planned in standard fashion in preparation for a mission), an automatic guidance mode consisting in making the aircraft follow this planned flight trajectory, in particular a navigation mode consisting more precisely in making the aircraft follow laterally a lateral trajectory of said planned flight trajectory, cannot be engaged in a managed mode, so long as the aircraft does not lie on said planned flight trajectory.
Also, when a pilot enables such an automatic guidance mode, in particular the aforesaid navigation mode, he must firstly guide the aircraft manually or according to a selected mode (that is to say by manually tuning particular parameters which are controlled on the aircraft) so as to allow him to join up with said planned flight trajectory, before said automatic guidance mode is engaged, this naturally requiring a significant work load of the pilot and giving rise to a delay in the engagement of the automatic guidance.
It will be noted that the fact of deviating from a planned flight trajectory is relatively frequent during a low altitude flight carried out in the military context, for example by a military transport plane, and generally depends on the operational conditions encountered and in particular on the search during the flight for the most effective possible terrain masking.